The traditional Celtic year was quartered by the solar events with which we are familiar—the
solstices (“sun-standing”) and the equinoxes (“equal-night”)—and then it was quartered
again at the midpoints between the solar events, the “cross-quarter days.” The result was that the
summer solstice, the longest day of the year, was midsummer’s day, not the first day of
summer as we observe it. The same goes for the winter solstice, the shortest day of the year
and the middle of winter in the old system. (It never made sense to me that as soon as summer
begins, the days start getting shorter, or that it has already been cold for some time by the “first day of
winter.”)

The solar events are determined astronomically, and vary a little bit due to factors such as leap year.
The true cross-quarter days are actually a few days later than the traditional dates of Imbolc, Beltaine,
Lughnasadh, and Samhain, and sources vary as to the exact dates of their observance as well.

The names of most of the old holidays are no longer household words (except Yule), and most people, if they have
heard the old names at all, are likely to associate them with paganism. Modern “pagans” who claim
these holidays as their own do nothing to dispel this idea. But they live on in
traditions such as Candlemas/Groundhog Day, May Day, and All Saint's Day/Halloween.